Thin film transistor, display device, and electronic unit

ABSTRACT

A thin film transistor using oxide semiconductor for a channel, which may be controlled such that threshold voltage is positive and may be improved in reliability is provided. The thin film transistor includes a gate electrode, a pair of source/drain electrodes, an oxide semiconductor layer forming a channel and provided between the gate electrode and the pair of source/drain electrodes, a first insulating film as a gate insulating film provided on the oxide semiconductor layer on a side near the gate electrode, and a second insulating film provided on the oxide semiconductor layer on a side near the pair of source/drain electrodes. One or both of the first insulating film and the second insulating film includes an aluminum oxide having a film density of 2.70 g/cm 3  or more and less than 2.79 g/cm 3 .

CROSS REFERENCES TO RELATED APPLICATIONS

The present application claims priority to Japanese Patent ApplicationJP 2010-090729 filed on Apr. 9, 2010, the entire contents of which ishereby incorporated by reference.

BACKGROUND

The present disclosure relates to a thin film transistor using oxidesemiconductor for a channel layer, and a display device and anelectronic unit, which use the thin film transistor.

Recently, research and development of oxide semiconductor such as zincoxide or indium-gallium-zinc oxide have been actively conducted with theaim of applying the oxide semiconductor to electronic devices such as athin film transistor (TFT), a light emitting device and a transparentconductive film. As generally known, when such oxide semiconductor isused for an active layer (channel) of TFT, the TFT has high electronmobility and thus has an excellent electric characteristic compared withTFT using amorphous silicon, which is typically used for a liquidcrystal display or the like. In addition, the TFT may be advantageouslyexpected to have high mobility even at low temperature near roomtemperature, and therefore the TFT is being actively developed. As suchTFT using the oxide semiconductor layer, TFT having a bottom-gate ortop-gate structure has been reported (for example, see WO2005-088726).

A known bottom-gate TFT is structured such that a gate electrode isprovided on a substrate, and a thin film layer of oxide semiconductor isformed on the gate electrode via a gate insulating film (for example,see Japanese Unexamined Patent Application Publication No. 2007-194594).Such a structure is similar to a structure of currently commerciallyused, bottom-gate TFT using amorphous silicon for a channel. Therefore,an existing manufacturing process of the TFT using amorphous silicon maybe easily used for manufacture of TFT using oxide semiconductor, andtherefore commercialization of the TFT using oxide semiconductor for achannel is gradually progressing.

However, since the oxide semiconductor is not high in heat resistance,oxygen or zinc may be eliminated during heat treatment in amanufacturing process of TFT, resulting in formation of lattice defects,as generally known. The lattice defects cause electrically shallowimpurity levels, leading to reduction in resistance of the oxidesemiconductor layer. Therefore, use of oxide semiconductor for a channelof TFT leads to normally-on operation, where certain drain current flowsthough gate voltage is not applied, or depression operation.Consequently, threshold voltage is reduced with increase in defectlevels, leading to increase in leakage current. Furthermore, asgenerally known, similar impurity levels are caused by mixing of aparticular element such as hydrogen in addition to the above impuritylevels caused by lattice defects (for example, see Cetin Kilic et. al.“N-type Doping of Oxides by Hydrogen” APPLIED PHYSICS LETTERS, 81, 1,2002, pp. 73-75).

Therefore, a transfer characteristic of TFT has been disadvantageouslychanged during a manufacturing process or the like, leading to shift ofthreshold voltage of the TFT in a negative (minus) direction.

For example, when oxide semiconductor is used to form an n-type channel,electron concentration in the channel increases, as a result, thresholdvoltage of TFT tends to have a negative value. For the TFT using oxidesemiconductor, since a p-type channel is hard to be formed, only n-typeTFT needs to be used for circuit formation. In such a case, when thethreshold voltage has a negative value, a circuit configuration becomesundesirably complicated.

SUMMARY

As a method to overcome such a difficulty, impurity doping is tried in apart of a channel of TFT on an interface between the channel and a gateinsulating film of the TFT in order to shift threshold voltage of theTFT (for example, see Japanese Unexamined Patent Application Publication(Translation of PCT Application) No. 2007-519256).

However, impurity doping in a channel may degrade TFT characteristics.Moreover, a channel of oxide semiconductor typically includes amulti-element material deposited by sputtering. When impurity doping ina channel is performed by the sputtering, element ratio control in thechannel has been extremely difficult.

It is desirable to provide a thin film transistor using oxidesemiconductor for a channel, which may be controlled such that thresholdvoltage is positive and may be improved in reliability, and provide adisplay device and an electronic unit, which use such thin filmtransistors.

A thin film transistor according to an embodiment includes a gateelectrode, a pair of source/drain electrodes, an oxide semiconductorlayer forming a channel and provided between the gate electrode and thepair of source/drain electrodes, a first insulating film as a gateinsulating film provided on the oxide semiconductor layer on a side nearthe gate electrode, and a second insulating film provided on the oxidesemiconductor layer on a side near the pair of source/drain electrodes,wherein one or both of the first insulating film and the secondinsulating film includes aluminum oxide having a film density of 2.70g/cm³ or more and less than 2.79 g/cm³.

A display device according to an embodiment includes display elementsand the thin film transistors.

An electronic unit according to an embodiment includes display elementsand the thin film transistors.

In the thin film transistor according to the embodiment, the firstinsulating film (gate insulating film) is provided on the oxidesemiconductor layer on the side near the gate electrode, and the secondinsulating film is provided thereon on the side near the source/drainelectrodes, and one or both of the first and second insulating filmsincludes aluminum oxide having a film density of 2.70 g/cm³ or more andless than 2.79 g/cm³. Such an insulating film has negative fixed chargeand thus negatively charged.

According to the thin film transistor of the embodiment, the insulatingfilm adjacent to the oxide semiconductor layer is formed of aluminumoxide having a film density of 2.70 g/cm³ or more and less than 2.79g/cm³, and therefore the insulating film has negative fixed charge thatmay shift threshold voltage of the transistor in a positive direction.Accordingly, while oxide semiconductor is used for a channel, thresholdvoltage may be controlled to be positive and reliability may beimproved.

Additional features and advantages are described herein, and will beapparent from the following Detailed Description and the figures.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 is a section diagram showing a section structure of a thin filmtransistor according to a first embodiment.

FIG. 2 is a CV characteristic diagram of three kinds of aluminum oxidefilms having different densities.

FIG. 3 is a characteristic diagram showing a relationship betweendensity and Vfb of aluminum oxide.

FIGS. 4A to 4C are diagrams showing a method of manufacturing the thinfilm transistor shown in FIG. 1 in a step order.

FIGS. 5A to 5C are diagrams showing steps following FIG. 4C.

FIG. 6 is a transfer characteristic diagram of thin film transistors ofan example and a comparative example.

FIG. 7 is a section diagram showing a section structure of a thin filmtransistor according to modification 1.

FIG. 8 is a section diagram showing a section structure of a thin filmtransistor according to a second embodiment.

FIG. 9 is a section diagram showing a section structure of a thin filmtransistor according to modification 2.

FIG. 10 is a block diagram showing a configuration example of a displaydevice having TFT.

FIG. 11 is a circuit diagram showing a detailed configuration example ofa pixel shown in FIG. 10.

FIG. 12 is a plan diagram showing a schematic configuration of a moduleincluding the display device shown in FIG. 10.

FIG. 13 is a perspective diagram showing appearance of applicationexample 1 of the display device shown in FIG. 10.

FIGS. 14A and 14B are perspective diagrams, where FIG. 14A showsappearance of application example 2 as viewed from a surface side, andFIG. 14B shows appearance thereof as viewed from a back side.

FIG. 15 is a perspective diagram showing appearance of applicationexample 3.

FIG. 16 is a perspective diagram showing appearance of applicationexample 4.

FIGS. 17A to 17G are diagrams of application example 5, where FIG. 17Ais a front diagram of the application example 5 in an opened state, FIG.17B is a side diagram thereof, FIG. 17C is a front diagram thereof in aclosed state, FIG. 17D is a left side diagram thereof, FIG. 17E is aright side diagram thereof, FIG. 17F is a top diagram thereof, and FIG.17G is a bottom diagram thereof.

DETAILED DESCRIPTION

Embodiments will be described in detail with reference to drawingsbelow. Description is made in the following sequence.

Bottom-Gate TFT

1. First embodiment (example of forming channel protective film usinglow-density aluminum oxide (Al₂O₃))

2. Modification 1 (example of gate insulating film having three-layerstructure with second layer of gate insulating film using low-densityAl₂O₃)

Top-Gate TFT

3. Second embodiment (example of forming basecoat film using low-densityAl₂O₃)

4. Modification 2 (example of forming gate insulating film usinglow-density Al₂O₃)

5. Application examples (examples of display device and electronicunits)

First Embodiment Configuration of Thin Film Transistor 1

FIG. 1 shows a section structure of a thin film transistor 1 accordingto a first embodiment. The thin film transistor 1 is a so-calledbottom-gate (inverted-staggered-structure) TFT, which uses oxidesemiconductor for a channel (an active layer). The thin film transistor1 includes a gate electrode 12, a gate insulating film 13, an oxidesemiconductor layer 14, a channel protective film 16 and source/drainelectrodes 15A and 15B formed in this order on a substrate 11 includingglass or the like. A protective film 17 is formed on the source/drainelectrodes 15A and 15B over the whole surface of the substrate 11. Thegate insulating film 13 corresponds to a specific example of “firstinsulating film”, and the channel protective film 16 corresponds to aspecific example of “second insulating film”.

The gate electrode 12 controls carrier density (here, electron density)in the oxide semiconductor layer 14 according to gate voltage applied tothe thin film transistor 1. The gate electrode 12 includes asingle-layer film including, for example, one of molybdenum (Mo),aluminum (Al) and aluminum alloy, or a multilayer film including two ormore of them. The aluminum alloy includes, for example,aluminum-neodymium alloy.

The gate insulating film 13 is a single-layer film including one of asilicon oxide film, a silicon nitride film, a silicon oxynitride film,and an aluminum oxide film, or a multilayer film including two or moreof them. Here, the gate insulating film 13 has a two-layer structureincluding a first layer 13A and a second layer 13B, and the first layer13A is formed of, for example, a silicon oxide film and the second layer13B is formed of, for example, a silicon nitride film. Thickness of thegate insulating film 13 is, for example, 200 to 300 nm.

The oxide semiconductor layer 14 mainly contains oxide of one or more ofelements of, for example, indium (In), gallium (Ga), zinc (Zn), tin(Sn), aluminum and titanium (Ti). The oxide semiconductor layer 14 formsa channel between the source and drain electrodes 15A and 15B inresponse to application of the gate voltage. The oxide semiconductorlayer 14 desirably has such a thickness that on-current of the thin filmtransistor is not reduced so that effects of negative charge asdescribed later may be exerted on the channel. Specifically, thethickness is desirably 5 to 100 nm.

The source/drain electrodes 15A and 15B include single-layer filmincluding, for example, one of molybdenum, aluminum, copper (Cu),titanium, ITO (indium-tin oxide) and titanium oxide, or a multilayerfilm including two or more of them. For example, metal or a metalcompound having low affinity for oxygen is desirably used for theelectrodes, such as a three-layer film including molybdenum, aluminumand molybdenum stacked in this order in a thickness of, for example, 50nm, 500 nm and 50 nm respectively, or a metal compound containing oxygensuch as ITO or titanium oxide. Consequently, an electric characteristicof oxide semiconductor may be stably maintained. On the other hand, whenthe source/drain electrodes 15A and 15B are formed of metal having highaffinity for oxygen, since the electrodes 15A and 15B are formed incontact with the oxide semiconductor, oxygen is extracted from oxidesemiconductor, and thus oxygen vacancies are formed, leading todegradation of the electric characteristic.

The channel protective film 16 is formed on the oxide semiconductorlayer 14 to prevent damage of a channel during formation of thesource/drain electrodes 15A and 15B. Thickness of the channel protectivefilm 16 is, for example, 10 to 300 nm. In the embodiment, the channelprotective film 16, which is in contact with the oxide semiconductorlayer 14, is formed of low-density aluminum oxide. Generally, aluminumoxide is used not only for the channel protective film 16 but also foran insulating film of a thin film transistor such as a gate insulatingfilm (Japanese Unexamined Patent Application Publication No.2007-258223). Such insulating films need to have high dielectricstrength. For an aluminum oxide film, higher dielectric strength isobtained in a denser (higher-density) film. Therefore, a high-densityaluminum oxide film has been used for a thin film transistor in thepast. In contrast, negative fixed-charge density tends to increase inthinner (lower-density) aluminum oxide film. In the embodiment, alow-density aluminum oxide film is used to shift threshold voltage ofthe thin film transistor in a positive direction. A specific density ofthe aluminum oxide film is preferably less than 2.79 g/cm³. A lowerlimit of the density is 2.70 g/cm³ in the light of the limit ofapparatus used for deposition of the aluminum oxide film. Morepreferably, the density is 2.75 g/cm³ or more and less than 2.79 g/cm³.The reason for this is described below.

FIG. 2 shows CV characteristics of three kinds of aluminum oxide filmshaving different densities. The CV characteristics are obtained throughmeasurement of the aluminum oxide films using a mercury prober, thefilms being deposited on a p-type silicon wafer at different depositionconditions. The densities of the aluminum oxide films are adjusted bycontrolling a deposition condition such as temperature. In FIG. 2, AlO-1indicates a high-density (2.82 g/cm³) aluminum oxide film, which istypically used as an insulating film, formed at 200° C. and DC power of11 kW. AlO-2 and AlO-3 indicate low-density aluminum oxide filmscompared with AlO-1, where AlO-2 is formed at 80° C. and DC power of 11kW, and AlO-3 is formed at 80° C. and DC power of 18 kW. As acomparative example, a CV characteristic of a silicon oxide film is alsoshown as another insulating film for oxide semiconductor TFT. Thesilicon oxide film is formed by PECVD (Plasma Enhanced CVD).

As shown in FIG. 2, when flat-band voltages (Vfb) of AlO-1, AlO-2 andAlO-3 are compared to Vfb of silicon oxide, Vfb of AlO-1 exists on anegative side with respect to the Vfb of silicon oxide. On the otherhand, Vfb of AlO-2 and Vfb of AlO-3 exist on a positive side withrespect to the Vfb of silicon oxide. Whether threshold voltage (Vth) ofa thin film transistor is shifted in a positive direction or in anegative direction may be estimated from a value of Vfb. Therefore, itis taught that AlO-2 or AlO-3 having low density compared with AlO-1 isused, thereby Vth of the thin film transistor is shifted in a positivedirection. In addition, it is taught that Vth of the thin filmtransistor may be shifted in a positive direction with respect to Vth ofa thin film transistor using silicon oxide.

FIG. 3 shows a relationship between a density and Vfb of each of thealuminum oxide films AlO-1, AlO-2 and AlO-3 deposited at the aboveconditions. As shown in FIG. 3, Vfb increases with decrease in densityof the aluminum oxide film. Therefore, an aluminum oxide film isadjusted to have a density lower than 2.82 g/cm³ corresponding todensity of AlO-1 as a typically used aluminum oxide film, thereby Vfb ofthe film increases, namely, threshold voltage of the thin filmtransistor is shifted in a positive direction. Furthermore, on the basisof the fact that Vfb of the silicon oxide film as another typicalinsulating film is −4.6 V, density of the aluminum oxide film isadjusted to be less than 2.79 g/cm³ corresponding to a density at acrossing of a line of an approximation formula obtained from respectiveVfb of AlO-1, AlO-2 and AlO-3 and a line of Vfb of the silicon oxidefilm, and therefore the threshold voltage of the thin film transistor isfurther shifted in a positive direction.

As described above, density of the aluminum oxide film needs to be lessthan 2.79 g/cm³ in order to achieve an enhancement-mode thin filmtransistor (Vth>0). However, such a lower-density aluminum oxide film islow in barrier capability. Therefore, a density gradient is preferablyprovided in a thickness direction of the aluminum oxide film in such amanner that a low-density film is deposited in tens of nanometers on anear-channel side, and other film portions are deposited to have highdensity in order to achieve the enhancement mode and the barriercapability together. When the density gradient is not provided, densityof the aluminum oxide film is preferably within a range of 2.75 g/cm³ ormore and less than 2.79 g/cm³ in view of balance between the enhancementmode and the barrier capability.

The low-density aluminum oxide film may be used not only for the channelprotective film 16 but also for the gate insulating film 13. However,when the thin film transistor is driven, since a certain electric fieldis applied to the gate insulating film 13, charge trapping may occur atan interface between the gate insulating film 13 and the channel (oxidesemiconductor layer 14), causing hysteresis. From this point of view,the low-density aluminum oxide film is preferably used for the channelprotective film 16.

The protective film 17 is formed of, for example, a single layer film ofan aluminum oxide film or a silicon oxide film, or a multilayer film ofan aluminum oxide film and a silicon oxide film. The aluminum oxide filmused herein is a high-density aluminum oxide film typically used for athin film transistor. Thickness of the protective film 17 is, forexample, 10 to 100 nm, and preferably 50 nm or less. In the oxidesemiconductor film, an electric characteristic of the film isinconveniently changed due to mixing of hydrogen or adsorption of water.However, the high-density aluminum oxide film is used as the protectivefilm 17, so that the adverse effect of hydrogen or water may beprevented by excellent gas-barrier capability of the film. In addition,the aluminum oxide film is used as the protective film 17, and thereforethe protective film may be formed without degrading an electriccharacteristic of oxide semiconductor.

Method of Manufacturing Thin Film Transistor 1

FIGS. 4 and 5 are diagrams for illustrating a method of manufacturingthe thin film transistor 1. The thin film transistor 1 may bemanufactured, for example, in the following way.

First, as shown in FIG. 4A, a metal thin film is deposited on the wholesurface of the substrate 11 by a sputtering or evaporation method, andthen the metal thin film is patterned by, for example, aphotolithography method to form the gate electrode 12.

Next, as shown in FIG. 4B, the second layer 13B and the first layer 13Aare sequentially deposited by, for example, a plasma CVD method so as tocover the substrate 11 and the gate electrode 12, so that the gateinsulating film 13 is formed. Specifically, first, the second layer 13Bincluding a silicon nitride film is deposited by a plasma CVD methodusing a mixed gas containing silane (SiH₄), ammonia (NH₃) and nitrogen.Then, the first layer 13A including a silicon oxide film is deposited bya plasma CVD method using a mixed gas containing silane and dinitrogenmonoxide (N₂O) as a source gas.

Next, as shown in FIG. 4C, the oxide semiconductor layer 14 is depositedby, for example, a sputtering method. Specifically, whenindium-gallium-zinc oxide (IGZO) is used as the oxide semiconductor, DCsputtering is performed with IGZO ceramics as a target. Here, it ispreferable that for example, a vacuum chamber of a DC sputteringapparatus is evacuated to, for example, 1×10⁻⁴ Pa or lower, and then amixed gas of argon (Ar) and oxygen is introduced into the chamber forplasma discharge. Carrier concentration in the channel may be controlledby adjusting a flow rate of argon to oxygen of the mixed gas.

Alternatively, when zinc oxide is used as the oxide semiconductor, RFsputtering can be performed with zinc oxide ceramics as a target, or DCsputtering can be performed with zinc as a target in a mixed gasatmosphere of argon and oxygen. Then, the oxide semiconductor layer 14is patterned in a desired shape by, for example, a photolithographymethod.

Next, as shown in FIG. 5A, the channel protective film 16 includingaluminum oxide having negative fixed charge is deposited on the oxidesemiconductor layer 14 by, for example, DC sputtering using Al as atarget. Here, it is preferable that for example, a vacuum chamber of aDC sputtering apparatus is evacuated to, for example, 1×10⁻⁴ Pa orlower, and then a mixed gas of argon (Ar) and oxygen is introduced intothe chamber for plasma discharge. As density of the aluminum oxide filmforming the channel protective film 16 decreases, density of negativefixed charge of the film increases, so that threshold voltage of TFT maybe more shifted in a positive direction. The density of the aluminumoxide film may be decreased by increasing DC power or decreasingtemperature during deposition. Moreover, since the amount of fixedcharge varies depending on thickness of the film, the threshold voltagemay be controlled by changing the thickness depending on desiredcharacteristics.

Next, as shown in FIG. 5B, the channel protective film 16 is patternedin a desired shape by, for example, a photolithography method.

Next, as shown in FIG. 5C, a metal thin film including, for example,molybdenum, aluminum and molybdenum stacked in this order is depositedby, for example, a sputtering method on the oxide semiconductor layer 14in a region including the channel protective film 16. Then, the metalthin film is patterned by a wet etching method using a mixed solutioncontaining phosphoric acid, nitric acid and acetic acid. Since thechannel protective film 16 protects a surface (channel surface) of theoxide semiconductor layer 14, the layer 14 may be prevented from beingdamaged by etching. Consequently, the source/drain electrodes 15A and15B are formed.

Next, the protective film 17 is formed by depositing, for example, analuminum oxide film on the source/drain electrodes 15A and 15B by, forexample, a sputtering method or an atomic layer deposition (ALD) method,and therefore the thin film transistor 1 shown in FIG. 1 is completed.

Operation and Effects of Thin Film Transistor 1

Next, operation and effects of the thin film transistor 1 of theembodiment are described.

In the thin film transistor 1, when a gate voltage equal to or higherthan a predetermined threshold voltage is applied to the gate electrode12 through a not-shown wiring layer, a channel is formed in the oxidesemiconductor layer 14, and thus electric current (drain current) flowsbetween the source and drain electrodes 15A and 15B, so that thetransistor 1 is activated.

In the embodiment, the channel protective film 16, including an aluminumoxide film having a density of 2.70 g/cm³ or more and less than 2.79g/cm³, is provided on the oxide semiconductor layer 14 (on a side nearthe source/drain electrodes 15A and 15B). The aluminum oxide film havinga density of less than 2.79 g/cm³ is used as the channel protective film16, so that the film 16 has negative fixed charge and thus negativelycharged. Consequently, the threshold voltage of the thin film transistor1 is shifted in a positive direction.

In this way, in the embodiment, the aluminum oxide film having a densityof 2.70 g/cm³ or more and less than 2.79 g/cm³ is used as the channelprotective film 16 provided on the oxide semiconductor layer 14, andtherefore the film 16 has negative fixed charge that may shift thethreshold voltage in a positive direction. Accordingly, the thin filmtransistor 1 using the oxide semiconductor for a channel may becontrolled such that threshold voltage is sifted in a positivedirection.

Example

As an example of the first embodiment, a transfer characteristic of TFT(a relationship between gate voltage and drain current) is measured.First, a transfer characteristic of TFT is measured for a channelprotective film 16 of an aluminum oxide film formed into a thickness of200 nm by DC sputtering at 80° C. and DC power of 18 kW (example), andmeasured for a channel protective film of a silicon oxide film formed bya plasma CVD method (comparative example). FIG. 6 shows measurements. Asshown, the transfer characteristic of TFT is shifted by about 0.8 V in apositive direction in the example using the channel protective film, inwhich density of the aluminum oxide film is reduced so as to generatenegative fixed charge, compared with the comparative example using thesilicon oxide film as the channel protective film. The transfercharacteristic may be controlled by adjusting density of the aluminumoxide film used as the channel protective film. Therefore, density ofthe aluminum oxide film is more decreased, so that the transfercharacteristic may be more shifted in a positive direction.

Modification 1

Next, a thin film transistor (thin film transistor 2) according to amodification (modification 1) of the thin film transistor of the firstembodiment is described. The thin film transistor 2 is bottom-gate TFTusing oxide semiconductor for a channel like the thin film transistor 1of the first embodiment. Hereinafter, the same elements as in the thinfilm transistor 1 of the first embodiment are designated by the samesymbols, and description of them is appropriately omitted.

FIG. 7 shows a section structure of the thin film transistor 2 accordingto the modification. The thin film transistor 2 includes a gateelectrode 12, a gate insulating film 18, an oxide semiconductor layer14, a channel protective film 19, source/drain electrodes 15A and 15B,and a protective film 17 formed in this order on a substrate 11 as inthe first embodiment. The gate insulating film 18 is a multilayer filmincluding, for example, three layers of a first layer 18A, a secondlayer 18B and a third layer 18C. Each of the first layer 18A, the secondlayer 18B and the third layer 18C includes a silicon oxide film, asilicon nitride film or an aluminum oxide film. Thicknesses of the firstlayer 18A, the second layer 18B and the third layer 18C are 300 nm, 50nm and 10 nm, respectively.

Such a gate insulating film 18 may be formed, for example, in thefollowing way. First, the first layer 18A including, for example, asilicon nitride film is formed by a plasma CVD method on the substrate11, having the gate electrode 12 formed thereon, in the same way as inthe embodiment. Then, a second layer 18B is formed on the first layer18A by, for example, DC sputtering using Al as a target. Next, the thirdlayer 18C including, for example, a silicon oxide film is formed by aplasma CVD method in the same way as in the embodiment. This results information of the gate insulating film 18 including the low-densityaluminum oxide film, having negative fixed charge, sandwiched by siliconoxide films or silicon nitride films. The oxide semiconductor layer 14is formed on the second layer 18B having negative fixed charge via thethird layer 18C. This may reduce hysteresis caused by charge trapping ata boundary between the aluminum oxide film and the oxide semiconductorfilm, the charge trapping occurring when the low-density aluminum oxidefilm and the oxide semiconductor layer 14 are directly stacked.

In the modification, the gate insulating film 18, formed on the oxidesemiconductor layer 14 on a side near the gate electrode 12, is madeinto a three-layer structure, and the low-density aluminum oxide filmhaving negative fixed charge is used for the second layer 18B, so thatthe gate insulating film 18 has negative fixed charge, and thusthreshold voltage may be shifted in a positive direction. Moreover,since the third layer 18C including silicon oxide is provided betweenthe second layer 18B including the low-density aluminum oxide film andthe oxide semiconductor layer 14, the hysteresis may be reduced.Accordingly, even if the gate insulating film 18, provided on the oxidesemiconductor layer 14 on the side near the gate electrode 12, is formedof the low-density aluminum oxide film, the same effects as in the firstembodiment may be obtained.

However, if a distance between the aluminum oxide film having negativefixed charge and the oxide semiconductor film increases, change inthreshold voltage is inconveniently reduced in addition to reduction inhysteresis. Therefore, the distance between the aluminum oxide film andthe oxide semiconductor film is preferably 5 to 10 nm.

While the first embodiment and the modification have been described withthe case where one of the channel protective film and the gateinsulating film of bottom-gate TFT is the low-density aluminum oxidefilm, both the films may be formed of the low-density aluminum oxidefilm.

Second Embodiment

FIG. 8 shows a section structure of a thin film transistor 3 accordingto a second embodiment. The thin film transistor 3 is so-called top-gate(staggered-structure) TFT, which uses oxide semiconductor for a channel.The thin film transistor 3 includes a base-coat film 20, source/drainelectrodes 15A and 15B, an oxide semiconductor layer 14, a gateinsulating film 13, and a gate electrode 12 formed in this order on asubstrate 11 including glass or the like. A protective film 17 is formedon the gate electrode 12 over the whole surface of the substrate 11. Inthe second embodiment, while a configuration relationship betweencomponents is different from the bottom-gate TFT described in the firstembodiment, since functions and materials of the components are thesame, the components are designated by the same symbols for convenience,and description of them is appropriately omitted.

In the embodiment, the base-coat film 20 formed on the substrate 11 is alow-density aluminum oxide film. The base-coat film 20 is provided toprevent mixing of impurities from a substrate 11, and is in contact withthe oxide semiconductor layer 14 via a separation groove between thesource and drain electrodes 15A and 15B formed on the film 20. In otherwords, the base-coat film 20 is formed in contact with a channel of theoxide semiconductor layer 14.

The low-density aluminum oxide film used as the base-coat film 20 isdeposited by, for example, DC sputtering using Al as a target. Here, itis preferable that, for example, a vacuum chamber of a DC sputteringapparatus is evacuated to, for example, 1×10⁻⁴ Pa or lower, and then amixed gas of argon (Ar) and oxygen is introduced into the chamber forplasma discharge. Density of the aluminum oxide film may be optionallyadjusted by controlling DC power or temperature during deposition. As aspecific density of the film, a density of less than 2.79 g/cm³ ispreferable as in the first embodiment. Thickness of the film is, forexample, 50 to 300 nm.

In this embodiment, the base-coat film 20, which is in contact with thechannel of the oxide semiconductor layer 14, includes the low-densityaluminum oxide, so that the base-coat film 20 has negative fixed charge,and thus threshold voltage is shifted in a positive direction.Accordingly, the same effects as in the first embodiment may beobtained.

Modification 2

FIG. 9 shows a section structure of a thin film transistor (thin filmtransistor 4) according to a modification (modification 2) of the secondembodiment. Even in the modification, while a configuration relationshipbetween components is different from the respective bottom-gate TFTdescribed in the first embodiment and the modification 1, sincefunctions and materials of the components are the same, the componentsare designated by the same symbols for convenience, and description ofthem is appropriately omitted.

The thin film transistor 4 is top-gate TFT using oxide semiconductor fora channel like the thin film transistor 3 of the second embodiment. Thethin film transistor 4 includes a base-coat film 21, source/drainelectrodes 15A and 15B, an oxide semiconductor layer 14, a gateinsulating film 18, and a gate electrode 12 formed in this order on asubstrate 11. The base-coat film 21 is formed of a silicon oxide film orthe like, and the gate insulating film 18 includes low-density aluminumoxide having negative fixed charge, and is deposited by, for example, DCsputtering using Al as a target. Process temperature may be lowered soas not to form the base-coat film 21.

In the modification, the gate insulating film 18, formed on the oxidesemiconductor layer 14 on a side near the gate electrode 12, includesthe low-density aluminum oxide, so that the gate insulating film 18 hasnegative fixed charge, and therefore threshold voltage is shifted in apositive direction. Accordingly, the same effects as in the firstembodiment may be obtained.

However, since a certain electric field is applied to the gateinsulating film 18, when low-density aluminum oxide is used, chargetrapping may occur at an interface between the oxide semiconductor layer14 and the gate insulating film 18, causing hysteresis. Thus, the gateinsulating film 18 is formed into a multilayer structure as in themodification 1, and an insulating film such as a silicon oxide film isprovided between the oxide semiconductor layer 14 and the insulatingfilm including the low-density aluminum oxide, and therefore thehysteresis may be reduced. In addition, thickness of the silicon oxidefilm is controlled to be 5 to 10 nm as in the modification 1, andtherefore while the hysteresis is suppressed, threshold voltage of thethin film transistor may be shifted in a positive direction at about thesame level as in the first embodiment. The gate insulating film 18 mayhave a two-layer structure. In such a case, the silicon oxide film isformed on a side near the oxide semiconductor film, and therefore thehysteresis may be reduced.

Application Examples

Next, description is made on application examples, to display devicesand electronic units, of the thin film transistors according to thefirst and second embodiments and the modifications 1 and 2.

Display Device

FIG. 10 shows a configuration example of a display device used as anorganic EL display (display device using organic EL elements). Forexample, the display device has a display region 30, in which aplurality of pixels PXLC including organic EL elements (organic fieldemission elements) as display elements are arranged in a matrix on a TFTsubstrate (the substrate 11). A horizontal selector (HSEL) 31 as asignal line driver circuit, a write scanner (WSCN) 32 as a scan linedriver circuit, and a drive scanner (DSCN) 33 as a drive line drivercircuit are provided in the periphery of the display region 30.

In the display region 30, a plurality of (an integer n) signal linesDTL1 to DTLn are arranged in a column direction, and a plurality of (aninteger m) scan lines WSL1 to WSLm and a plurality of (an integer m)drive lines DSL1 to DSLm are arranged in a row direction, respectively.Each pixel PXLC (one of pixels corresponding to red (R), green (G) andblue (B)) is provided at an intersection of each signal line DTL andeach scan line WSL. Each signal line DTL is connected to the horizontalselector 31 that supplies a video signal to each signal line DTL. Eachscan line WSL is connected to the write scanner 32 that supplies a scansignal (selection pulse) to each scan line WSL. Each drive line DSL isconnected to the drive scanner 33 that supplies a drive signal (controlpulse) to each drive line DSL.

FIG. 11 shows a circuit configuration example of the pixel PXLC. Eachpixel PXLC has a pixel circuit 40 including an organic EL element 3D.The pixel circuit 40 is an active driver circuit having a samplingtransistor 3A, a driver transistor 3B, a capacitance element 3C and theorganic EL element 3D. The transistors 3A and 3B correspond to the thinfilm transistor of each of the embodiments and the like.

A gate of the sampling transistor 3A is connected to a correspondingscan line WSL, and one of a source and a drain of the transistor isconnected to a corresponding signal line DTL while the other isconnected to a gate of the driver transistor 3B. A drain of the drivertransistor 3B is connected to a corresponding drive line DSL, and asource thereof is connected to an anode of the organic EL element 3D. Acathode of the organic EL element 3D is connected to a ground line 3H.The ground line 3H is connected in common to all the pixels PXLC. Thecapacitance element 3C is disposed between the source and the gate ofthe driver transistor 3B.

The sampling transistor 3A becomes conductive in response to a scansignal (selection pulse) supplied from a scan line WSL, and thus samplesa signal potential of a video signal supplied from a signal line DTL,and holds the signal potential in the capacitance element 3C. The drivertransistor 3B is supplied with current from a drive line DSL set to apredetermined first potential (not shown), and supplies to the organicEL element 3D a drive current in correspondence with the signalpotential held in the capacitance element 3C. The organic EL element 3Dis supplied with the drive current from the driver transistor 3B andthus emits light with luminance corresponding to the signal potential ofthe video signal.

In the display device, the sampling transistor 3A becomes conductive inresponse to a scan signal (selection pulse) supplied from a scan lineWSL, and thus a signal potential of a video signal supplied from asignal line DTL is sampled and held in the capacitance element 3C. Inaddition, current is supplied from a drive line DSL set to the firstpotential to the driver transistor 3B that supplies to the organic ELelement 3D (each of organic EL elements of red, green and blue) a drivecurrent in correspondence with the signal potential held in thecapacitance element 3C. Each organic EL element 3D is supplied with thedrive current and thus emits light with luminance corresponding to thesignal potential of the video signal. Consequently, the display deviceperforms video display based on the video signal.

Electronic Units

Hereinafter, application examples of the display device to electronicunits are described. The display device may be used for electronic unitsin any field, including a television apparatus, a digital camera, anotebook personal computer, a mobile terminal such as mobile phone, anda video camera. In other words, the display device may be used forelectronic units in any field for displaying still or video images basedon an externally-input or internally-generated video signal.

Module

The display device may be built in various electronic units such asthose in application examples 1 to 5 described below, for example, in aform of a module shown in FIG. 12. In the module, for example, a region210 exposed from a sealing substrate 50 is provided in one side of thesubstrate 11, and external connection terminals (not shown) are formedin the exposed region 210 by extending lines of the horizontal selector31, the write scanner 32 and the drive scanner 33. The externalconnection terminals may be attached with a flexible printed circuit(FPC) 220 for input or output of signals.

Application Example 1

FIG. 13 shows appearance of a television apparatus. The televisionapparatus has, for example, an image display screen 300 including afront panel 310 and filter glass 320, and the image display screen 300corresponds to the display device.

Application Example 2

FIGS. 14A and 14B show appearance of a digital camera. The digitalcamera has, for example, a light emitting section for flash 410, adisplay 420, a menu switch 430 and a shutter button 440, and the display420 corresponds to the display device.

Application Example 3

FIG. 15 shows appearance of a notebook personal computer. The notebookpersonal computer has, for example, a body 510, a keyboard 520 for inputoperation of letters and the like, and a display 530 for displayingimages, and the display 530 corresponds to the display device.

Application Example 4

FIG. 16 shows appearance of a video camera. The video camera has, forexample, a body 610, an object-shooting lens 620 provided on a frontside-face of the body 610, a start/stop switch 630 for shooting, and adisplay 640. The display 640 corresponds to the display device.

Application Example 5

FIGS. 17A to 17G show appearance of a mobile phone. For example, themobile phone is assembled by connecting an upper housing 710 to a lowerhousing 720 by a hinge 730, and has a display 740, a sub display 750, apicture light 760, and a camera 770. The display 740 or the sub display750 corresponds to the display device.

While the embodiments and the like have been described with the case asan example where a gate insulating film is a two-layer film of a siliconoxide film and a silicon nitride film, or a three-layer film including alow-density aluminum oxide film sandwiched by silicon oxide films orsilicon nitride films, the gate insulating film may be formed into asingle-layer structure or a multilayer structure including four or morelayers.

While the embodiments and the like have been described with the case asan example where the channel protective film using low-density aluminumoxide having negative fixed charge is in contact with the oxidesemiconductor layer 14, the channel protective film need not be fullycontacted on the layer 14. In other words, if the low-density aluminumoxide film having negative fixed charge exists at least near the oxidesemiconductor layer 14 as described in the modifications, the sameeffects as in the embodiments and the like may be obtained.

It should be understood that various changes and modifications to thepresently preferred embodiments described herein will be apparent tothose skilled in the art. Such changes and modifications can be madewithout departing from the spirit and scope of the present subjectmatter and without diminishing its intended advantages. It is thereforeintended that such changes and modifications be covered by the appendedclaims.

1. A thin film transistor comprising: a gate electrode; a sourceelectrode; a drain electrode; an oxide semiconductor layer forming achannel and provided between the gate electrode and the source and drainelectrodes; a first insulating film as a gate insulating film providedon the oxide semiconductor layer on a side near the gate electrode, anda second insulating film provided on the oxide semiconductor layer on aside near the source and drain electrodes, wherein one or both of thefirst insulating film and the second insulating film includes analuminum oxide having a film density of 2.70 g/cm³ or more and less than2.79 g/cm³.
 2. The thin film transistor according to claim 1, whereinthe one or both of insulating films are a single-layer film.
 3. The thinfilm transistor according to claim 1, wherein the one or both ofinsulating films have a two-layer structure, and one layer includessilicon oxide or silicon nitride, and the other layer includes thealuminum oxide.
 4. The thin film transistor according to claim 3,wherein the layer including aluminum oxide is stacked on the oxidesemiconductor layer via the layer including silicon oxide or siliconnitride.
 5. The thin film transistor according to claim 1, wherein theone or both of insulating films have a three-layer structure, and onelayer of the insulating film includes the aluminum oxide, and other twolayers include silicon oxide or silicon nitride, and sandwich the layerincluding the aluminum oxide.
 6. The thin film transistor according toclaim 1, wherein film density of the insulating film has a gradient in adepth direction, and the gradient of the film density is low on a sidenear the oxide semiconductor layer.
 7. A display device includingdisplay elements and thin film transistors for driving the displayelements, each of the thin film transistors comprising: a gateelectrode; a source electrode; a drain electrode; an oxide semiconductorlayer forming a channel and provided between the gate electrode and thesource and drain electrodes; a first insulating film as a gateinsulating film provided on the oxide semiconductor layer on a side nearthe gate electrode; and a second insulating film provided on the oxidesemiconductor layer on a side near the source and drain electrodes,wherein one or both of the first insulating film and the secondinsulating film includes aluminum oxide having a film density of 2.70g/cm³ or more and less than 2.79 g/cm³.
 8. An electronic unit having adisplay device which includes display elements and thin film transistorsfor driving the display elements, each of the thin film transistorscomprising: a gate electrode; a source electrode; a drain electrode; anoxide semiconductor layer forming a channel and provided between thegate electrode and the source and drain electrodes; a first insulatingfilm as a gate insulating film provided on the oxide semiconductor layeron a side near the gate electrode; and a second insulating film providedon the oxide semiconductor layer on a side near the source and drainelectrodes, wherein one or both of the first insulating film and thesecond insulating film includes aluminum oxide having a film density of2.70 g/cm³ or more and less than 2.79 g/cm³.